Materials change yields fast transparent circuits

Portland, Ore. -- A process used to create the world's first inexpensive transparent circuitry based on inorganic materials could enable a new era of electronics. The circuit--a five-inverter ring oscillator cast in amorphous indium gallium oxide--was recently demonstrated here at Oregon State University.

The transparent circuits could be "embossed" on virtually any surface, from "smart glass"--displays with integrated computers in a "windshield"--to ultraefficient solar-cell "coatings" painted onto an electric car.

"We now have proof that transparent transistors can be used to create an integrated circuit, and we showed how to make transparent circuits with conventional photolithography techniques," said John Wager, professor of electrical engineering at Oregon State University. "We are moving transparent electronics from the laboratory toward working commercial applications." Wager performed the work with fellow professors Doug Keszler and Janet Tate, and electrical engineer Rick Presley.

Since the process is inorganic, it promises superior performance compared with organic transparent circuits. The inorganic circuits show higher electron mobility, higher chemical stability and higher physical durability, but use only low-temperature fabrication that rivals those of conductive polymers.

Today, most research into transparent electronics centers on organic conductive polymers, which are very inexpensive to manufacture and which, unlike silicon circuitry, only require low temperatures for fabrication.

Process uses indium, gallium
In 2003, Wager's research group developed the world's first inorganic transparent transistor. It was fabricated from zinc-tin-oxide, and like organics, used only low-temperature processing. And last year, Tokyo Institute of Technology researchers showed that adding indium and gallium to zinc-tin-oxide enabled inorganic transparent transistors to use a polymer substrate (see www.eetasia.com/ARTP_8800356665_499489.HTM).

Now Oregon State University researchers have crafted a complete circuit with a process by which indium and gallium can be sputtered onto an amorphous substrate. The technique should rival the low cost of organic circuits while eliminating their need for caustic heavy metals like mercury, lead and arsenic, making this new approach to transparent circuitry environmentally friendly.

There are hurdles ahead, however, including fabricating p-channel devices to complement the just-demonstrated n-channel transistors. That would allow the researchers to create complementary metal-oxide semiconductors whose differential signals enable low-energy operation.

"All of the remaining work seems very feasible," said Wager. "We might be able to bring transparent integrated circuits into widespread use within five years."

The process is licensed to Hewlett-Packard Co. for manufacturing. The team is working with Hewlett-Packard to streamline the manufacturability of the process, provide physical protection for the delicate circuitry and scale up the density of devices.

Wager's research group was funded by the National Science Foundation, the Army Research Office and Hewlett-Packard.

The project was accomplished in cooperation with the Oregon Nanoscience and Microtechnologies Institute, a research group consisting of Oregon State University, Portland State University, the University of Oregon, the Pacific Northwest National Laboratory and the state of Oregon.